The present invention relates to a process for manufacturing oxirane by reaction between an olefin and a peroxide compound in the presence of a catalyst and a solvent. In particular, the invention relates to the manufacture of propylene oxide (or epichlorohydrin) by epoxidation of propylene (or allyl chloride) using hydrogen peroxide in the presence of a catalyst containing TS-1.
It is known practice to manufacture propylene oxide by reaction between propylene and hydrogen peroxide in the presence of TS-1. For example, in patent U.S. Pat. No. 5,849,937, such a process is performed in several reactors arranged in series. In this known process, each reactor of the series is fed with fresh hydrogen peroxide.
The Applicant has found that when each reactor is fed with fresh hydrogen peroxide, it is impossible to convert the entire amount of hydrogen peroxide used without substantial formation of by-products and thus impossible to have an optimum yield.
The present invention overcomes this drawback by providing a novel process which makes it possible to convert 100% of the amount of hydrogen peroxide introduced, with a low formation of by-products and without, however, reducing the reaction rate.
To this end, the invention relates to a process for manufacturing oxirane by reaction of an olefin with a peroxide compound in the presence of a catalyst and a solvent in at least two reactors arranged in series, each of which contains a portion of catalyst, according to which a first portion of the olefin, the solvent and all of the peroxide compound are introduced into a first reactor, an epoxidation of the first portion of the olefin is carried out therein in order to form a first portion of the oxirane, a medium comprising the first portion of the oxirane formed, the solvent, the unconsumed peroxide compound and possibly the unconverted olefin is removed from this reactor, the medium and another portion of the olefin are introduced into a subsequent reactor, an epoxidation of the other portion of the olefin is carried out therein using the unconsumed peroxide compound obtained from the first reactor, in order to form another portion of the oxirane, and the other portion of the oxirane thus formed is collected.
One of the essential characteristics of the present invention lies in the fact that the peroxide compound is only introduced into the first reactor. The subsequent reactor(s) is (are) thus not fed with fresh peroxide compound, but only with the peroxide compound which is present in the medium obtained from the preceding reactor and which has not been consumed in this preceding reactor. In general, water is also introduced with the peroxide compound into the first reactor. The fact that no peroxide compound is added to the subsequent reactor(s) makes it possible to consume 100% of the total amount of peroxide compound used without, however, reducing the reaction rate when compared with a process using the same total amount of peroxide compound, but in which each reactor is fed with fresh peroxide compound.
In the process according to the invention, a plant comprising at least two epoxidation reactors arranged in series and connected together is used. Each reactor is fed with olefin. The peroxide compound and the solvent are only introduced into the first reactor. Each reactor contains a portion of the catalyst which does not leave that reactor. When the catalyst is present in the form of a fixed bed, it is generally not necessary to take precautions to keep the catalyst in the reactor. Alternatively, the catalyst may be present in the form of particles, at least some of which are in a form fluidized by a liquid stream or by mechanical stirring or by a gas. When a liquid stream is used, it is recommended to include a fall-out zone above the fluid bed to stop the catalyst particles which are in motion and/or to include a filter at the reactor outlet.
Needless to say, the plant may comprise more than two reactors connected in series. In this case, the first reactor of the series is fed with the olefin, the peroxide compound and the solvent and each subsequent reactor is fed with the olefin and the medium obtained from the preceding reactor of the series. Preferably, 3 reactors in series are used.
In the process according to the invention, at least 50% of the total amount of peroxide compound used in the first reactor is generally consumed in the first reactor. The best yields are obtained when at least 70% are consumed in the first reactor. Usually, not more than 99% are consumed in the first reactor, and preferably not more than 85%. The remainder is consumed in the subsequent reactor(s).
In the process according to the invention, reactors of identical size are preferably used. This makes it possible to interchange the function of the reactors when the deactivated catalyst in one reactor is replaced with fresh or regenerated catalyst without disrupting the functioning of the plant (so-called xe2x80x9ccarouselxe2x80x9d functioning).
A first embodiment of the process according to the invention consists in using the catalyst in the form of particles, at least some of which are in fluidized form, as disclosed in the Applicant""s patent application filed on the same day as the present patent application and entitled xe2x80x9cProcess for manufacturing oxirane in the presence of a catalyst in the form of particlesxe2x80x9d (the content of which is incorporated by reference). In this case, it is recommended to include a filter through which the medium leaving the first reactor passes before being introduced into the subsequent reactor. This embodiment makes it possible to obtain a homogeneous dispersion of the catalyst in the epoxidation reaction medium, good heat exchange and thus easy control of the reaction temperature.
In a second embodiment of the process according to the invention, the medium entering the subsequent reactor is first subjected to a depressurization before being introduced into the subsequent reactor. This embodiment is particularly suitable when the epoxidation is carried out under pressure or in the presence of a gaseous compound. This gaseous compound may be the olefin itself (for example propylene) or an inert gas which is introduced into the epoxidation reaction medium to allow the oxirane to be entrained and removed from the reactor, as disclosed in patent application WO 99/48883 by the Applicant.
In a third embodiment of the process according to the invention, the medium entering the subsequent reactor is first subjected to a treatment to separate out the oxirane formed before being introduced into the subsequent reactor. The aim of this embodiment is to separate the oxirane from the epoxidation reaction medium as quickly as possible after it is formed in order to prevent the formation of by-products by hydrolysis or alcoholysis (methanolysis when methanol is used as solvent) of the oxirane formed. This embodiment thus has the advantage of leading to a high selectivity. The separation treatment is preferably a distillation, as disclosed in the Applicant""s patent application filed on the same day as the present patent application and entitled xe2x80x9cProcess for manufacturing oxirane comprising the separation of the oxirane from the reaction mediumxe2x80x9d (the content of which is incorporated by reference).